photonic processing for wideband cancellation and spectral discrimination...

Hindawi Publishing CorporationAdvances in Optical TechnologiesVolume 2013 Article ID 738427 8 pageshttpdxdoiorg1011552013738427

Research ArticlePhotonic Processing for Wideband Cancellation and SpectralDiscrimination of RF Signals

David M Benton

L-3 TRL Technology Unit 19 Miller Court Severn Drive Tewkesbury Gloucestershire GL20 8DN UK

Correspondence should be addressed to David M Benton davidbentonl-3comcom

Received 9 July 2013 Revised 16 October 2013 Accepted 16 October 2013

Academic Editor Augusto Belendez

Copyright copy 2013 David M Benton This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Photonic signal processing is used to implement common mode signal cancellation across a very wide bandwidth utilising phasemodulation of radio frequency (RF) signals onto a narrow linewidth laser carrier RF spectra were observed using narrow-band tunable optical filtering using a scanning Fabry Perot etalon Thus functions conventionally performed using digital signalprocessing techniques in the electronic domain have been replaced by analog techniques in the photonic domain This techniquewas able to observe simultaneous cancellation of signals across a bandwidth of 1400MHz limited only by the free spectral range ofthe etalon

1 Introduction

Photonic signal processing offers a new paradigm for pro-cessing high bandwidth signals and opens up possibilities fordirectly processing signals that are modulated onto an opticalcarrier The ability to process and manipulate ultrawidebandsignals enables high-resolution reconfigurable processingof potentially the entire RF spectrum of signals It is thispotential that has received a lot of attention over the past fewdecades [1 2] and is suitably demonstrated through devicessuch as wideband adaptivemicrowave filters enabled throughcoherence and phase control [3] Photonic technologies area natural source for delivering increased bandwidth butalso the low loss of fibre optic transmission systems hasseen developments in transmitting Radio over Fibre (RoF)and antenna remoting [4 5] As requirements for increaseddata bandwidth develop and as spectral densities increasewith congestion becoming an issue the need for ever morepowerful digital signal processing (DSP) is a constant driverThis has inevitable consequences for size weight and power(SWaP) considerations especially in RF signal monitoringand characterisation equipment With the RF signal in pho-tonic form and the inherent bandwidth benefits that ensuethere is a natural opportunity for implementing analog signalprocessing (ASP) techniques and making the processing aninherent part of the optical link [6]

Signal cancellation is conventionally implemented byengineering a version of the target waveform that is matchedin amplitude but 180∘ out of phase (negative feedback) suchthat the interference of the target and engineered waveformsresult in cancellation This approach can be performed withanalog or digital signals but is likely to be limited in itsoperational bandwidth by frequency dependent phase shiftsPhotonic carriers modulated by RF signals are not limitedby these frequency dependent phase shifts in circuitry andhence can operate across a much wider bandwidth Photonicsignal processing (the manipulation of light waves ratherthan digitised signals) can be used to cancel out commonwideband RF signals enabling weaker signals to be identifiedWhilst cancellation is common in photonic signal processingit has tended to be targeted at sideband removal [7ndash10]Suppression of modulation signals in photonic systems withsome specific examples of signal cancellation was shownthrough inversion in an optical amplifier [11] Single sidebandsuppression is a common technique in which the signal ismade to interfere with a phase shifted version of itself helpingto remove the power residing in the (unmodulated) carriercomponent

There have recently been reports of photonic based RFcancellation systems that claim impressive levels of cancel-lation [12ndash15] typically 40 dB for broadband signals and a

2 Advances in Optical Technologies

very impressive 70 dB for single tones These systems repr-esent the current state of the art in photonic cancellationbut have significant differences from the system discussedin this paper Applications of this technique have focussedon removal of cosite interference where a close proximitytransmitter will pose significant problems for a receiverparticularly relevant where jammers are being used andcan overwhelm a receiver significantly reducing its abilityto observe weak signals However the pairwise nature ofsignal generation for cancellation does not specifically requireone signal to be the reference to be removed Where thetwo signal sources are independent cancellation will seek toreduce signal contributions that are common to both sourcesConversely signal contributions unique to one source onlywill be unaffected and can emphasise the differences betweenthe two sources which can help with signal gradiometry andsource location In this paper photonic processing techniques(equivalent to ASP) are implemented to demonstrate theproof of principle that not only the signal cancellation butalso spectral analysis across a wide bandwidth is attainableby optical means without significant DSP

2 Photonic Processing andOptical Cancellation

The cancellation of signals common to two independentsignal sources such as two antennas can be implemented bymodulating the RF signal from each antenna onto an opticalphase modulator each incorporated into one arm of a MachZehnder interferometer (MZI) and fed from a common laserConventional MZI modulators are widely available but areused for applications such as single sideband suppressionwhere only a single input signal is required and the outputis amplitude modulated Using separate phase modulators ineach arm allows independent signals to be combined andindependent phase control is available for each signal priorto coherent combination

The simplest demonstration is where a phase shift of 120587

is imposed upon one of the modulated laser fields by theapplication of a bias voltage and a photodiode detector canbe used to observe the resulting intensity after the two armsare recombinedThis setup is shown schematically in Figure 1for the simplest case

An electrooptic modulator subject to a sinusoidally oscil-lating RF electric field of modulation frequency 120596

119898gives rise

to an output field consisting of a series of sidebands centredaround the laser carrier frequency represented by (see eg[16])

119864 (119905) = 1198640

infin

sum

119896=0

119869119896

(120575) cos (120596 + 119896120596119898

) 119905

+

infin

sum

119896=0

(minus1)119896

119869119896

(120575) cos (120596 minus 119896120596119898

) 119905

(1)

where 119869119896(120575) represents a 119896th order Bessel function and 120575 is

a modulator characteristic parameter In the case above twomodulated fields 119864

1(119905) and 119864

2(119905) are produced in eachmodu-

lator respectively and then combinedWhere onemodulator

Laser 22 fibre

RF source 2

RF source 1

Detector

DC voltage

splitter

Figure 1 A schematic diagram for observing common modecancellation in the simplest case

has a 120587 phase change added to the carrier frequency 120596 thenall terms at frequency (120596+119896120596

119898) can be cancelled in principle

In this arrangement all frequency components must be mea-sured through conventional sampling and spectral powersdetermined through the Fourier transform

More generally the RF signals received at each modulatorwill have a phase difference due for example to time of flightdifferences from source to antennas The electric field exitingeach modulator can be represented in an ideal set up by

1198641

(119905) = 1198640cos (120596119905 + 120575 sin (120596

119898119905 + 120579)) (2)

1198642

(119905) = 1198640cos (120596119905 + 120601 + 120575 sin (120596

119898119905)) (3)

where 120579 is the signal phase offset between the received signals120601 is the optical phase offset applied to the carrier by themodulator and each modulator is assumed to receive thesame optical amplitude119864

0 After application of trigonometric

identities and ignoring harmonic terms above 119896 = 1 we canwrite for modulator (1)

1198641

(119905) = 1198640

[1198690

(120575) cos (120596119905) + 1198691

(120575) cos ((120596 minus 120596119898

) 119905 minus 120579)

minus1198691

(120575) cos ((120596 + 120596119898

) 119905 + 120579)]

(4)

Similarly from modulator (2)

1198642

(119905) = 1198640

[1198690

(120575) cos (120596119905 + 120601) + 1198691

(120575) cos ((120596 minus 120596119898

) 119905 + 120601)

minus1198691

(120575) cos ((120596 + 120596119898

) 119905 + 120601)]

(5)

The amplitudes from each modulator are combined withone arm experiencing a 120587 phase shift (due to being a MZI)

In the simple example above involving Nyquist samplingof the detector signal the amplitude at the modulationfrequency will be observed through the mixing within thedetector between the modulation signal and the unmodu-lated carrier Whilst this provides coherent gain to increasethe signal amplitude it prevents direct comparison of thesignal frequencies from each source and is prone to variationas the two carrier components being combined can becancelled independently from the modulation signals as thecarrier phase is varied It is also possible that a single sourceof modulation in only one modulator can appear to becancelled solely through its phase difference with the carrierTo overcome this the modulation components must be

Advances in Optical Technologies 3

DAQ

Optical

Fibre

RF source 2

RF source 1Phase modulators

Bias tee

RF splitter

Electrical connectionFibre optic connectionCoil of fibre

Offset voltage

Detector signal

DetectorData acquisition

Scanning etalon

splittercombiner

Fibresplittercombiner

isolator

and control system

Single frequencylaser 1550 nm

Etalon scancontroller

Experimental observations

Figure 2 System setup including a scanning Fabry Perot etalon

separated from the carrier Separating the carrier frequencyfrom the sidebands can in principle be achieved throughdispersion such as using a diffraction grating However amodulation component of 10MHz has a frequency differenceof only 1 part in 107 from the unmodulated carrier and wouldthus require an impractical propagation distance before themodulation component could be spatially separated from thecarrier Separation of the carrier andmodulation componentsis best achieved through filtering The Fabry Perot etalon(FPE) is in optical terms (though not in RF terms) a highresolution tunable filter formed of two reflective surfaces(mirrors) accurately separated by a known distance [17] Thetransmission through the filter can be tuned by adjustingthe spacing between the mirrors and the filter bandwidthwhich is determined by the level of reflectivity of the mirrorsFPEs with a bandwidth of 10MHz are commercially availableand capable of discriminating between the laser carrier andmodulation sidebands A photodiode detector placed afterthe FPE measures the intensity being transmitted and theparticular mirror separation during a scan determines thefrequency to which this intensity relates Thus the spectralcharacteristics of the modulation are measured by scanningthe FPE filter not by rapid temporal sampling and processingThe spectral bandwidth limitations are those of the FPE andare determined by the free spectral range which can be 10sof GHz It is also convenient to observe the intensity in boththe positive and negative sidebands if requiredThus spectralcharacteristics across an ultrawide bandwidth can be accessedwithout recourse to very high speed DSP

After filtering with the FPE the observed intensity ineach sideband arises from the time average of the square ofthe total amplitude from both modulators In the positive

sideband120596+120596119898 the observed time averaged intensity ismade

up of the positive sideband contributions in (4) and (5)

⟨119864 (120596 + 120596119898

)2

⟩ = ⟨1198642

0

(minus1198691

(120575))2

times [cos ((120596 + 120596119898

) 119905 + 120579)

+ cos ((120596 + 120596119898

) 119905 + 120601 + 120587)]2

⟩

= 1198642

0

1198691

(120575)2

⟨1 minus cos (120579 minus 120601)⟩

(6)

Similarly for the negative sideband 120596 minus 120596119898

⟨119864 (120596 minus 120596119898

)2

⟩ = 1198642

0

1198691

(120575)2

⟨1 minus cos (120579 + 120601)⟩ (7)

Thuswith a suitable choice of carrier phase offset120601 frequencycomponents common to both modulators can be minimisedin one or both of the sidebands Hence the ldquoprocessingrdquoof cancellation and spectral transform are achieved throughonly optical means

3 Experimental Observations

An experimental setup was constructed using commerciallyavailable optical fibre based components and is shown inFigure 2 A narrow linewidth laser (100KHz) operating at1550 nm with up to 40mW of power was split into two armsusing a 2 2 single mode fibre splitter each of which wasconnected to a fibre coupled electrooptic phase modulatorThe modulated outputs from the phase modulators wererecombined with the use of a 2 1 splittercombiner The out-put from the recombiner was collimated and then focussed

4 Advances in Optical TechnologiesPh

ase o

ffset

var

ying

with

tim

e

Carrierfrequency

+400 MHzcommon

signal

+500 MHzuniquesignal

minus500 MHzuniquesignal

minus400 MHzcommon

signal

Figure 3 A time evolving spectral plot (waterfall plot) showing pos-itive and negative sidebands around the central carrier frequency A400MHz signal is common to both modulators whilst a 500MHzsignal is present in only one modulator and is unaffected by phasechanges

into a scanning Fabry Perot etalon (FPE) The transmittedoutput intensity was measured with an amplified photodiodesampled with an ADC within a data acquisition unit (DAQ)Data was collected and processed using a LabView programRF signals were supplied from a signal generator and directedto each modulator via a splitter A DC voltage was controlledfrom the LabView program and applied to one modulatorvia a bias tee The LabView program monitored the spectralamplitude at the signal frequency as the DC voltage wassystematically varied thereby varying the relative carrierphase between the two arms

A scanning FPE with a free spectral range of 15 GHz anda finesse of 100 was incorporated into the system This waschosen primarily to give a reasonable compromise betweenbandwidth (wide in RF terms) resolution (poor in RF terms)and physical size Whilst significantly higher finesse etalonscould be used they are large and cumbersome devices notwellsuited to use outside of the laboratoryThus for any system tobe widely useful in the real world size matters The etalonscan was set to cover a spectral range of 750MHz either sideof the laser carrier frequency so as to prevent confusion withsidebands from the neighbouring free spectral range thus theeffective spectral range is 750MHzThe scan controller cycledthe etalon spacing at a rate of 100Hz and a detector followingthe etalon measured the spectral intensity in relation to thefilter position The RF signal amplitude spectral componentsare related to the optical amplitude seen by the detector Thefrequency is determined by the filter position and not byany temporal modulation thus the spectrum can be obtainedwith a very low rate sampler whilst covering a wide spectralrange

The detector was sampled at a rate of 10 kSps and relatedto the scan position using timing gates issued from theetalon scan controller A time evolving spectral plot (waterfallplot) could be produced to examine the evolution of spectralamplitudes with time and hence with applied voltage (phase)

A common signal at 400MHz (5 dBm) was applied to bothmodulators via a set of splitters Another ldquouniquerdquo signal at500MHz was applied to just one of the modulators The plotof Figure 3 shows how the amplitudes of the two signals varywith time as phase changes In this case a step change wasmade to the applied voltage and the system was then left tostabilise The phase varied slowly with time due to thermaleffects and the common 400MHz signal is affected by thisin both sidebands The carrier frequency amplitude is alsoaffected by this phase drift but the unique 500MHz signalremains unaffected thus demonstrating that signal present inonly one arm can be discriminated from pervasive commonsignals

Some characteristics of this plot need explanation Firstlythe upper and lower sidebands at 400MHz are out of phasein their response to the phase change In this case this arisesfrom a difference in the length of the cables connecting thesignal generator to the modulators equivalent to a time offlight phase difference in the modulation signal Secondlythe carrier power is cancelled at a different phase to thesidebands This arises due to a lack of polarisation definitionbefore the laser light enters the modulators Modulationoccurs predominantly for one polarisation state within themodulator and withoutmatching the input polarisation thereexist two ldquomodesrdquomdashone modulated and one unmodulatedmdashin the output fieldThe coherent addition of the unmodulatedcarrier mode and the modulated (119869

0(120575)) carrier mode results

in a carrier field with an offset phase relative to the sidebandsThis situation will be rectified in future with the use ofmodulatorswith integral polarisation stagesNevertheless theproposition that common mode signals can be cancelled isadequately shown here During the course of these investi-gations it was observed that the phase applied to the carrierwas not linear with applied voltage This was determined tobe due to ohmic heating in the modulator devices whichare impedance matched for use at 10GHzThe thermal effectupon the refractive index of the modulator material results ina nonlinear phase response as outlined in the appendix

To demonstrate that this cancellation can be carriedout across a wide instantaneous bandwidth a programmablesignal generator was used to repeatedly scan the commonmodulation frequency in steps of 20MHz between 100MHzand 600MHz whilst a unique frequency of 500MHz wasmaintained in one of the modulators The offset voltagewas then adjusted for maximum suppression of the com-mon signal In this case the cable length connected to themodulators was matched so that no frequency dependentmodulation arising from the RF offset phase was presentThe data showing the effect of common signal suppressionacross a 500MHz bandwidth is shown in Figure 4 where thecancellation level here is 11 dB across the entire frequencyrange simultaneously In this case simultaneous cancellationoccurs due to zero relative phase (120579 = 0) of the signals ateach modulator thus (6) and (7) are minimised for the samecarrier phase

The amount of cancellation seen here is limited due toa number of contributions In particular the central carrierfrequency is many orders of magnitude more intense thanthe sidebands and its tail adds noise to the sideband spectral


200 400 600

1

2

Frequency (MHz)

Phot

odio

de si

gnal

(V)

No cancellation

Tim

e (10

0 ms s

teps

)

0

2

4

6

(a)

200 400 600

1

2

Frequency (MHz)

Phot

odio

de si

gnal

(V)

With cancellation

Tim

e (10

0 ms s

teps

)

0

2

4

6

(b)

Figure 4 Data plots showing one sideband with a frequency scanning signal in both modulators and unique signal at 500MHz in only onemodulator (a) Simultaneous cancellation of common mode signals across a 500MHz bandwidth can be seen in the data on (b)

locations Residual signal can be seen in the suppressed dataconfirming that cancellation is not complete The lack ofpolarisation stage before the modulators leads to differencesin the amplitude of the modulated mode which will limit theamount of cancellation that is possibleThis can be addressedsomewhat by variable attenuators placed after themodulatorsbut this leads to a reduction in overall signal The isolationoffered by the FPE is also imperfect and this adds to thenoise level Modelling has also exposed that the cancellationlevel is very sensitive to the precise amount of phase offsetapplied In this particular case the voltage step resolutioncorresponded to a phase change of around 05∘ which wouldlimit the cancellation level to 30 dB and hence clearly othercontributions were the dominant limitation to the amountof cancellation achieved Whilst the demonstrated level ofcancellation is a little disappointing it is valuable whenconsidering that this system has implemented the dual rolesof cancellation and spectral transform without any recourseto high speed sampling or DSPThe cancellation system setupapplies equally across a very wide bandwidth limited here bythe free spectral range of the FPE but in principle can operateacross the bandwidth of the modulator Only an adjustmentofmodulator offset voltage is required to tune the cancellationfor any given frequency and offset signal phase Cancellationacross a range 100ndash1500MHz is shown in Figure 5 wherehigher frequencies are seen entering from right to left andoriginate from a neighbouring spectral order There is much

potential for improving the performance of this system insubsequent development

4 Sideband Behaviour

Both upper and lower sidebands can be observed (as seenin Figure 3) and the behaviour of the spectral power in eachsideband is dependent upon the relative RF signal phasedifference in the two modulators and the carrier phase Theunique signal is however unaffected by the carrier phase andtherefore cancellation of common signal can be observed inwhatever sideband is most beneficial

If each modulator is subject to an RF signal obtained viaan antenna in a general setting the distribution of sourcesin the local environment is likely to be uncontrollable Thereceived signals will then be subject to a phase differencedependent upon the signal frequency and the spacing of thereceiving antennas Thus cancellation will require a carrierphase setting specific for that source It can be seen from (6)and (7) that whatever the phase difference 120579 of the modula-tion signal a value of carrier offset phase can be chosen thatwill result in cancellation of that modulation signal in eithersideband This can be equivalently seen when a mismatchin cable lengths between signal generator and modulators isintroduced which introduces a frequency dependent phaseshift A cable of length 1m was introduced before one


Frequency (MHz) Frequency (MHz)

(a) (b)

With cancellationNo cancellation

0

2

1

0

2

1

600400200 600400200

Unique signal at 600 MHz

100ndash750 MHz 100ndash750 MHz

750ndash1500 MHz

750ndash1500 MHz

Tim

e (10

0 ms s

teps

)

Tim

e (10

0 ms s

teps

)

Figure 5 A wideband frequency scanning signal into both modulators with a unique frequency at 500MHz Time increases down thediagram and frequencies 100ndash750MHz proceed left to right whilst 750ndash1500MHz from neighbouring spectral order proceed right to leftCancellation is demonstrated in (b)

minus05

0

05

1

15

2

minus700 minus500 minus300 minus100 100 300 500 700

Frequency (MHz)Measured sideband intensityPhase dependent envelope

Figure 6 Frequency dependent sideband intensity variations inupper and lower sidebands for a fixed path length difference en-route to the modulators

modulator The frequency dependent phase difference 120579(119891)

is then

120579 (119891) =

119889120576119903

1003816100381610038161003816119891

1003816100381610038161003816

119888

(8)

where 119889 is the path difference and 120576119903is the relative permit-

tivity of the cable Data was captured encompassing bothsidebands where a sequentially varying frequency input waspresented to the modulators The strong saturated centralcarrier component was removed from the data by fittinga Lorentzian lineshape (with a half-width of 19MHz andamplitude of 7592) Frequency calibration was carried outwith a linear fit to each sideband independentlyThe envelopeof the sideband data was then modelled using the expectedfrequency dependent phase and is shown in Figure 6 In thiscase the modelling revealed that the carrier offset phase wasset to 60∘ which could not be determined independently dueto the thermal effects upon the modulators

5 Improvements and Extensions

Subsequent developments have shown that sharing the offsetvoltage across both modulators in a push-pull configura-tion improves stability significantly for two reasons Firstlywhilst the electrooptic phase change is parity dependent thethermal phase change is not hence the thermal effects inboth modulators have a similar size and the differential effectupon the output phase from the pair of modulators is muchreduced Also because the magnitude of the applied voltageto each modulator is half the required voltage the powerdissipated in each modulator is reduced by a relative factorof 4 A relative test applying a 2V offset change (from 0V)was applied comparing application to a singlemodulator withapplication across the pair This has seen the thermal phasedrift reduced from 4∘s when applied to a single modulatorto 08∘s in a push pull-configuration with the settling timereduced from 200 s to 50 s

In this paper the intended application has been thecomparison of signals from two separate antennas withthe capability of removing signals common to both Inother photonic cancellation work [12 13] the usage has beenpredominantly removing a known transmission signal whichcan be easily obtained and used to provide the feedbackreference signal Using 2 signals from spatially separatedantenna would clearly introduce a signal dependent phaseoffset which is related to the source location relative to theantenna and is the source of the 120579 term in (6) and (7)Therefore to observe cancellation effects for any particularsignal the carrier phase offset must be adjusted and itis straightforward to conceive of capturing spectra whilstscanning the carrier phase Indeed given the uncontrollablenature of source locations in the real world it is necessaryto make measurements with at least 2 phase values in orderto observe changes in the amount of cancellation takingplaceThese two measurements should differ by 180∘ to allowthe possibility of maximum and minimum cancellation to


be observed Knowing the source frequency the separationof the receiving antennas and relative phase obtained fromcomparing the sideband strengths allows two possibilitiesfor the angle of arrival to be determined and a measureof source location to be performed Unambiguous sourcedirection finding would require 3 receivers Preliminaryattempts have shown an angle-of-arrival influence upon thesideband strengths but this requires more investigation anddevelopment

6 Conclusion

This work has demonstrated the basic principle that photonic(analog) processing can be used to implement widebandsignal cancellation between two signal sources modulatedonto coherent laser carriers using phase modulation Furtherto this photonic processing can additionally be used to imple-ment frequency spectrum generation across an ultrawidebandwidth without recourse to high speed sampling thusreplacing the digital signal processing required to performa Fourier transform In the case given here filtering is usedto separate the sidebands from the carrier in order thatunambiguous cancellation effects can be observed but thisis due in part to an inadequacy of the phase modulatorsused here In principle significant carrier cancellation canalso take place which will help in reducing noise in the finalfiltered output by reducing the height of the Lorentzian tailof the residual carrier These are improvements that can beincorporated in future developments all of which shouldhelp to increase the level of cancellation observed at theoutput detectorThe system is suitable for incorporation ontoan integrated waveguide system which would significantlyreduce the system size and complexityThe current limitationto the system performance is the Fabry Perot etalon wherethe finesse of 100 limits the isolation between carrier powerand sidebands and the FSR is a limit to the unambiguousbandwidth that can be observed Etalons with higher finesseand FSR are available but are large bulky and cumbersomeand hence not well suited to usage outside of laboratoryconditions There are clearly a number of developments thatwhen implemented could lead to significant improvementsin performance and would form part of a developmentprogramme Nevertheless this is step towards analog signalprocessing for spectral analysis

This proof of concept has shown that it is feasible toconsider the general case of common mode cancellationof signals across a very wide bandwidth rather than justthe specific case of a cancellation of a single known signalAll that is required is the control of a single voltage levelapplied to a phase modulator enabling phase scanning andthe subsequent observation of the spectrum will reveal therelative phases of common signal components Further tothis there is significance in the observation that signals thatare not common to both receivers are unaffected by thecancellation process This coupled with observations of thesideband intensities on the positive and negative sides canreveal information about the spatial distribution of commonsources Converting RF signals into the photonic domain can

therefore offer attractive benefits in terms of reducing DSPrequirements and acquiring signal source information

Appendix

The phase offset Δ120601 from an electrooptic modulator has 2contributions the electrooptic effect Δ120601EO and a thermaleffect Δ120601

119879

Δ120601 = Δ120601EO + Δ120601119879

=

2120587119891119871

119888

(Δ119899EO + Δ119899119879

) (A1)

where Δ119899 is a change in refractive index 119891 is the appliedfrequency 119871 is the length of the modulator and 119888 is the speedof light

The electrooptic effect gives a refractive index change

Δ119899EO =

1198993

0

119903 119881 sdot 119871

2119889

(A2)

where 119903 is the electrooptic tensor value for the material 119881 isthe applied voltage 119889 is the electrode gap and 119871 is the lengthof the crystal

The change in temperature Δ119879 of the device is related tothe amount of energy 119876 deposited through the well-knownrelation

119876 = 119898119862Δ119879 (A3)

where 119898 is the mass of the device and 119862 is the specific heatcapacity

The energy deposited is proportional to the square of theapplied voltage 119876 prop (119881

2

119877) leading to

Δ119899119879

=

119889119899

119889119879

Δ119879 =

119889119899

119889119905

120581

1198812

119898119862119877

(A4)

where 120581 is a constantThus the phase change has the form

Δ120601 =

2120587119891119871

119888

(

1198993

0

11990333

119881 sdot 119871

2119889

+

119889119899

119889119905

120581

1198812

119898119862119877

) (A5)

For LiNbO3

11990333= 30 pmV 119889119899119889119879 = 37 times 10minus6∘C

This form will have the effect of reducing the spacingbetween consecutive voltages providing a 120587 phase offset Theprecise device characteristics are not known and so accuratemodelling cannot be performed However estimating theelectrode gap to be of order 10 120583m the EO component isof the order 119899

3

0

times 10minus7 which is similar in scale to 119889119899119889119879This broadly agrees with the voltage squared dependenceobserved but the net result is that thermal effects causeslowly varying phase instability until thermal equilibrium isreached

References

[1] A J Seeds ldquoMicrowave photonicsrdquo IEEETransactions onMicro-wave Theory and Techniques vol 50 no 3 pp 877ndash887 2002


[2] R A Minasian ldquoPhotonic signal processing of microwave sig-nalsrdquo IEEE Transactions on Microwave Theory and Techniquesvol 54 no 2 pp 832ndash846 2006

[3] F Zeng and J Yao ldquoInvestigation of phase-modulator-based all-optical bandpass microwave filterrdquo Journal of Lightwave Tech-nology vol 23 no 4 pp 1721ndash1728 2005

[4] E I Ackerman andA S Daryoush ldquoBroad-band externalmod-ulation fiber-optic links for antenna-remoting applicationsrdquoIEEE Transactions onMicrowaveTheory and Techniques vol 45no 8 pp 1436ndash1442 1997

[5] DNovak T Clark S OrsquoConnor D Oursler and RWaterhouseldquoHigh performance compact RF photonic transmitter withfeedforward linearizationrdquo in Proceedings of the IEEE MilitaryCommunications Conference (MILCOM rsquo10) pp 880ndash884October-November 2010

[6] T K Woodward T C Banwell A Agarwal P Toliver and RMenendez ldquoSignal processing in analog optical linksrdquo in Proc-eedings of the IEEE Avionics Fiber-Optics and Photonics Tech-nology Conference (AVFOP rsquo09) pp 17ndash18 September 2009

[7] M Y Frankel and R D Esman ldquoOptical single-sideband sup-pressed-carrier modulator for wide-band signal processingrdquoJournal of Lightwave Technology vol 16 no 5 pp 859ndash863 1998

[8] A Loayssa J M Salvide D Benito and M J Garde ldquoNoveloptical single-sideband suppressed-carrier modulator using abidirectionally-driven electro-optic modulatorrdquo in Proceedingsof the International Topical Meeting on Microwave Photonics(MWP rsquo00) 2000

[9] A Loayssa J M Salvide D Benito and M J Garde ldquoNoveloptical single-sideband suppressed-carrier modulator using abidirectionally-driven electro-optic modulatorrdquo in Proceedingsof the International Topical Meeting on Microwave Photonics(MWP rsquo00) pp 117ndash120 2000

[10] A Siahmakoun SGranieri andK Johnson ldquoDouble and singleside-band suppressed-carrier optical modulator implementedat 1320 nm using LiNbO3 crystals and bulk opticsrdquo in Opto-electric andWireless Data Management Processing Storage andRetrieval vol 4534 of Proceedings of SPIE pp 86ndash92 August2001

[11] F Coppinger S Yegnanarayanan P D Trinh and B Jalali ldquoAll-optical rf filter using amplitude inversion in a semiconductoroptical amplifierrdquo IEEE Transactions on Microwave Theory andTechniques vol 45 no 8 pp 1473ndash1477 1997

[12] M Alemohammad D Novak and R Waterhouse ldquoPhotonicsignal cancellation for co-site interference mitigationrdquo in Pro-ceedings of the IEEEMilitary Communications Conference (MIL-COM rsquo11) pp 2142ndash2146 November 2011

[13] M Lu J Bruno Y Deng P R Prucnal and A Hofmaier ldquoCo-site interference mitigation using optical signal processingrdquo inEnabling Photonics Technologies for Defense Security andAerospace Applications VIII vol 8397 of Proceedings of SPIEJune 2012

[14] J Suarez K Kravtsov and P R Prucnal ldquoIncoherent method ofoptical interference cancellation for radio-frequency communi-cationsrdquo IEEE Journal of Quantum Electronics vol 45 no 4 pp402ndash408 2009

[15] J Suarez andP R Prucnal ldquoSystem-level performance and char-acterization of counter-phase optical interference cancellationrdquoJournal of Lightwave Technology vol 28 no 12 pp 1821ndash18312010

[16] A Yariv Optical Electronics chapter 9 HRW international 3rdedition 1985

[17] E Hecht Optics chapter 9 Addison-Wesley Reading MassUSA 1987

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VLSI Design

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very impressive 70 dB for single tones These systems repr-esent the current state of the art in photonic cancellationbut have significant differences from the system discussedin this paper Applications of this technique have focussedon removal of cosite interference where a close proximitytransmitter will pose significant problems for a receiverparticularly relevant where jammers are being used andcan overwhelm a receiver significantly reducing its abilityto observe weak signals However the pairwise nature ofsignal generation for cancellation does not specifically requireone signal to be the reference to be removed Where thetwo signal sources are independent cancellation will seek toreduce signal contributions that are common to both sourcesConversely signal contributions unique to one source onlywill be unaffected and can emphasise the differences betweenthe two sources which can help with signal gradiometry andsource location In this paper photonic processing techniques(equivalent to ASP) are implemented to demonstrate theproof of principle that not only the signal cancellation butalso spectral analysis across a wide bandwidth is attainableby optical means without significant DSP

2 Photonic Processing andOptical Cancellation

The cancellation of signals common to two independentsignal sources such as two antennas can be implemented bymodulating the RF signal from each antenna onto an opticalphase modulator each incorporated into one arm of a MachZehnder interferometer (MZI) and fed from a common laserConventional MZI modulators are widely available but areused for applications such as single sideband suppressionwhere only a single input signal is required and the outputis amplitude modulated Using separate phase modulators ineach arm allows independent signals to be combined andindependent phase control is available for each signal priorto coherent combination

The simplest demonstration is where a phase shift of 120587

is imposed upon one of the modulated laser fields by theapplication of a bias voltage and a photodiode detector canbe used to observe the resulting intensity after the two armsare recombinedThis setup is shown schematically in Figure 1for the simplest case

An electrooptic modulator subject to a sinusoidally oscil-lating RF electric field of modulation frequency 120596

119898gives rise

to an output field consisting of a series of sidebands centredaround the laser carrier frequency represented by (see eg[16])

119864 (119905) = 1198640

infin

sum

119896=0

119869119896

(120575) cos (120596 + 119896120596119898

) 119905

+

infin

sum

119896=0

(minus1)119896

119869119896

(120575) cos (120596 minus 119896120596119898

) 119905

(1)

where 119869119896(120575) represents a 119896th order Bessel function and 120575 is

a modulator characteristic parameter In the case above twomodulated fields 119864

1(119905) and 119864

2(119905) are produced in eachmodu-

lator respectively and then combinedWhere onemodulator

Laser 22 fibre

RF source 2

RF source 1

Detector

DC voltage

splitter

Figure 1 A schematic diagram for observing common modecancellation in the simplest case

has a 120587 phase change added to the carrier frequency 120596 thenall terms at frequency (120596+119896120596

119898) can be cancelled in principle

In this arrangement all frequency components must be mea-sured through conventional sampling and spectral powersdetermined through the Fourier transform

More generally the RF signals received at each modulatorwill have a phase difference due for example to time of flightdifferences from source to antennas The electric field exitingeach modulator can be represented in an ideal set up by

1198641

(119905) = 1198640cos (120596119905 + 120575 sin (120596

119898119905 + 120579)) (2)

1198642

(119905) = 1198640cos (120596119905 + 120601 + 120575 sin (120596

119898119905)) (3)

where 120579 is the signal phase offset between the received signals120601 is the optical phase offset applied to the carrier by themodulator and each modulator is assumed to receive thesame optical amplitude119864

0 After application of trigonometric

identities and ignoring harmonic terms above 119896 = 1 we canwrite for modulator (1)

1198641

(119905) = 1198640

[1198690

(120575) cos (120596119905) + 1198691

(120575) cos ((120596 minus 120596119898

) 119905 minus 120579)

minus1198691

(120575) cos ((120596 + 120596119898

) 119905 + 120579)]

(4)

Similarly from modulator (2)

1198642

(119905) = 1198640

[1198690

(120575) cos (120596119905 + 120601) + 1198691

(120575) cos ((120596 minus 120596119898

) 119905 + 120601)

minus1198691

(120575) cos ((120596 + 120596119898

) 119905 + 120601)]

(5)

The amplitudes from each modulator are combined withone arm experiencing a 120587 phase shift (due to being a MZI)

In the simple example above involving Nyquist samplingof the detector signal the amplitude at the modulationfrequency will be observed through the mixing within thedetector between the modulation signal and the unmodu-lated carrier Whilst this provides coherent gain to increasethe signal amplitude it prevents direct comparison of thesignal frequencies from each source and is prone to variationas the two carrier components being combined can becancelled independently from the modulation signals as thecarrier phase is varied It is also possible that a single sourceof modulation in only one modulator can appear to becancelled solely through its phase difference with the carrierTo overcome this the modulation components must be


DAQ

Optical

Fibre

RF source 2


Bias tee

RF splitter


Offset voltage

Detector signal


Scanning etalon

splittercombiner


isolator

and control system









⟨119864 (120596 + 120596119898

)2

⟩ = ⟨1198642

0

(minus1198691

(120575))2

times [cos ((120596 + 120596119898

) 119905 + 120579)

+ cos ((120596 + 120596119898

) 119905 + 120601 + 120587)]2

⟩

= 1198642

0

1198691

(120575)2

⟨1 minus cos (120579 minus 120601)⟩

(6)


⟨119864 (120596 minus 120596119898

)2

⟩ = 1198642

0

1198691

(120575)2

⟨1 minus cos (120579 + 120601)⟩ (7)





ase o

ffset

var

ying

with

tim

e

Carrierfrequency

+400 MHzcommon

signal



minus400 MHzcommon

signal












200 400 600

1

2

Frequency (MHz)

Phot

odio

de si

gnal

(V)

No cancellation

Tim

e (10

0 ms s

teps

)

0

2

4

6

(a)

200 400 600

1

2

Frequency (MHz)

Phot

odio

de si

gnal

(V)

With cancellation

Tim

e (10

0 ms s

teps

)

0

2

4

6

(b)









(a) (b)


0

2

1

0

2

1

600400200 600400200



750ndash1500 MHz

750ndash1500 MHz

Tim

e (10

0 ms s

teps

)

Tim

e (10

0 ms s

teps

)


minus05

0

05

1

15

2





is then

120579 (119891) =

119889120576119903

1003816100381610038161003816119891

1003816100381610038161003816

119888

(8)








6 Conclusion




Appendix


119879

Δ120601 = Δ120601EO + Δ120601119879

=

2120587119891119871

119888

(Δ119899EO + Δ119899119879

) (A1)



Δ119899EO =

1198993

0

119903 119881 sdot 119871

2119889

(A2)



119876 = 119898119862Δ119879 (A3)



2

119877) leading to

Δ119899119879

=

119889119899

119889119879

Δ119879 =

119889119899

119889119905

120581

1198812

119898119862119877

(A4)


Δ120601 =

2120587119891119871

119888

(

1198993

0

11990333

119881 sdot 119871

2119889

+

119889119899

119889119905

120581

1198812

119898119862119877

) (A5)

For LiNbO3

11990333= 30 pmV 119889119899119889119879 = 37 times 10minus6∘C


3

0


References




















VLSI Design



RotatingMachinery





Shock and Vibration



Advances in







Journal of





SensorsJournal of








Journal of










Volume 2014

RoboticsJournal of



DAQ

Optical

Fibre

RF source 2


Bias tee

RF splitter


Offset voltage

Detector signal


Scanning etalon

splittercombiner


isolator

and control system









⟨119864 (120596 + 120596119898

)2

⟩ = ⟨1198642

0

(minus1198691

(120575))2

times [cos ((120596 + 120596119898

) 119905 + 120579)

+ cos ((120596 + 120596119898

) 119905 + 120601 + 120587)]2

⟩

= 1198642

0

1198691

(120575)2

⟨1 minus cos (120579 minus 120601)⟩

(6)


⟨119864 (120596 minus 120596119898

)2

⟩ = 1198642

0

1198691

(120575)2

⟨1 minus cos (120579 + 120601)⟩ (7)





ase o

ffset

var

ying

with

tim

e

Carrierfrequency

+400 MHzcommon

signal



minus400 MHzcommon

signal












200 400 600

1

2

Frequency (MHz)

Phot

odio

de si

gnal

(V)

No cancellation

Tim

e (10

0 ms s

teps

)

0

2

4

6

(a)

200 400 600

1

2

Frequency (MHz)

Phot

odio

de si

gnal

(V)

With cancellation

Tim

e (10

0 ms s

teps

)

0

2

4

6

(b)









(a) (b)


0

2

1

0

2

1

600400200 600400200



750ndash1500 MHz

750ndash1500 MHz

Tim

e (10

0 ms s

teps

)

Tim

e (10

0 ms s

teps

)


minus05

0

05

1

15

2





is then

120579 (119891) =

119889120576119903

1003816100381610038161003816119891

1003816100381610038161003816

119888

(8)








6 Conclusion




Appendix


119879

Δ120601 = Δ120601EO + Δ120601119879

=

2120587119891119871

119888

(Δ119899EO + Δ119899119879

) (A1)



Δ119899EO =

1198993

0

119903 119881 sdot 119871

2119889

(A2)



119876 = 119898119862Δ119879 (A3)



2

119877) leading to

Δ119899119879

=

119889119899

119889119879

Δ119879 =

119889119899

119889119905

120581

1198812

119898119862119877

(A4)


Δ120601 =

2120587119891119871

119888

(

1198993

0

11990333

119881 sdot 119871

2119889

+

119889119899

119889119905

120581

1198812

119898119862119877

) (A5)

For LiNbO3

11990333= 30 pmV 119889119899119889119879 = 37 times 10minus6∘C


3

0


References




















VLSI Design



RotatingMachinery





Shock and Vibration



Advances in







Journal of





SensorsJournal of








Journal of










Volume 2014

RoboticsJournal of



ase o

ffset

var

ying

with

tim

e

Carrierfrequency

+400 MHzcommon

signal



minus400 MHzcommon

signal












200 400 600

1

2

Frequency (MHz)

Phot

odio

de si

gnal

(V)

No cancellation

Tim

e (10

0 ms s

teps

)

0

2

4

6

(a)

200 400 600

1

2

Frequency (MHz)

Phot

odio

de si

gnal

(V)

With cancellation

Tim

e (10

0 ms s

teps

)

0

2

4

6

(b)









(a) (b)


0

2

1

0

2

1

600400200 600400200



750ndash1500 MHz

750ndash1500 MHz

Tim

e (10

0 ms s

teps

)

Tim

e (10

0 ms s

teps

)


minus05

0

05

1

15

2





is then

120579 (119891) =

119889120576119903

1003816100381610038161003816119891

1003816100381610038161003816

119888

(8)








6 Conclusion




Appendix


119879

Δ120601 = Δ120601EO + Δ120601119879

=

2120587119891119871

119888

(Δ119899EO + Δ119899119879

) (A1)



Δ119899EO =

1198993

0

119903 119881 sdot 119871

2119889

(A2)



119876 = 119898119862Δ119879 (A3)



2

119877) leading to

Δ119899119879

=

119889119899

119889119879

Δ119879 =

119889119899

119889119905

120581

1198812

119898119862119877

(A4)


Δ120601 =

2120587119891119871

119888

(

1198993

0

11990333

119881 sdot 119871

2119889

+

119889119899

119889119905

120581

1198812

119898119862119877

) (A5)

For LiNbO3

11990333= 30 pmV 119889119899119889119879 = 37 times 10minus6∘C


3

0


References




















VLSI Design



RotatingMachinery





Shock and Vibration



Advances in







Journal of





SensorsJournal of








Journal of










Volume 2014

RoboticsJournal of



200 400 600

1

2

Frequency (MHz)

Phot

odio

de si

gnal

(V)

No cancellation

Tim

e (10

0 ms s

teps

)

0

2

4

6

(a)

200 400 600

1

2

Frequency (MHz)

Phot

odio

de si

gnal

(V)

With cancellation

Tim

e (10

0 ms s

teps

)

0

2

4

6

(b)









(a) (b)


0

2

1

0

2

1

600400200 600400200



750ndash1500 MHz

750ndash1500 MHz

Tim

e (10

0 ms s

teps

)

Tim

e (10

0 ms s

teps

)


minus05

0

05

1

15

2





is then

120579 (119891) =

119889120576119903

1003816100381610038161003816119891

1003816100381610038161003816

119888

(8)








6 Conclusion




Appendix


119879

Δ120601 = Δ120601EO + Δ120601119879

=

2120587119891119871

119888

(Δ119899EO + Δ119899119879

) (A1)



Δ119899EO =

1198993

0

119903 119881 sdot 119871

2119889

(A2)



119876 = 119898119862Δ119879 (A3)



2

119877) leading to

Δ119899119879

=

119889119899

119889119879

Δ119879 =

119889119899

119889119905

120581

1198812

119898119862119877

(A4)


Δ120601 =

2120587119891119871

119888

(

1198993

0

11990333

119881 sdot 119871

2119889

+

119889119899

119889119905

120581

1198812

119898119862119877

) (A5)

For LiNbO3

11990333= 30 pmV 119889119899119889119879 = 37 times 10minus6∘C


3

0


References




















VLSI Design



RotatingMachinery





Shock and Vibration



Advances in







Journal of





SensorsJournal of








Journal of










Volume 2014

RoboticsJournal of




(a) (b)


0

2

1

0

2

1

600400200 600400200



750ndash1500 MHz

750ndash1500 MHz

Tim

e (10

0 ms s

teps

)

Tim

e (10

0 ms s

teps

)


minus05

0

05

1

15

2





is then

120579 (119891) =

119889120576119903

1003816100381610038161003816119891

1003816100381610038161003816

119888

(8)








6 Conclusion




Appendix


119879

Δ120601 = Δ120601EO + Δ120601119879

=

2120587119891119871

119888

(Δ119899EO + Δ119899119879

) (A1)



Δ119899EO =

1198993

0

119903 119881 sdot 119871

2119889

(A2)



119876 = 119898119862Δ119879 (A3)



2

119877) leading to

Δ119899119879

=

119889119899

119889119879

Δ119879 =

119889119899

119889119905

120581

1198812

119898119862119877

(A4)


Δ120601 =

2120587119891119871

119888

(

1198993

0

11990333

119881 sdot 119871

2119889

+

119889119899

119889119905

120581

1198812

119898119862119877

) (A5)

For LiNbO3

11990333= 30 pmV 119889119899119889119879 = 37 times 10minus6∘C


3

0


References




















VLSI Design



RotatingMachinery





Shock and Vibration



Advances in







Journal of





SensorsJournal of








Journal of










Volume 2014

RoboticsJournal of




6 Conclusion




Appendix


119879

Δ120601 = Δ120601EO + Δ120601119879

=

2120587119891119871

119888

(Δ119899EO + Δ119899119879

) (A1)



Δ119899EO =

1198993

0

119903 119881 sdot 119871

2119889

(A2)



119876 = 119898119862Δ119879 (A3)



2

119877) leading to

Δ119899119879

=

119889119899

119889119879

Δ119879 =

119889119899

119889119905

120581

1198812

119898119862119877

(A4)


Δ120601 =

2120587119891119871

119888

(

1198993

0

11990333

119881 sdot 119871

2119889

+

119889119899

119889119905

120581

1198812

119898119862119877

) (A5)

For LiNbO3

11990333= 30 pmV 119889119899119889119879 = 37 times 10minus6∘C


3

0


References




















VLSI Design



RotatingMachinery





Shock and Vibration



Advances in







Journal of





SensorsJournal of








Journal of










Volume 2014

RoboticsJournal of




















VLSI Design



RotatingMachinery





Shock and Vibration



Advances in







Journal of





SensorsJournal of








Journal of










Volume 2014

RoboticsJournal of


photonic processing for wideband cancellation and spectral discrimination...

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